In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used in 1980's. Reducing the wavelength of exposure light was believed effective as the means for further reducing the feature size. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) in 1990's and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source was required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the postponement of F2 lithography and the early introduction of ArF immersion lithography were advocated (see Proc. SPIE Vol. 4690 xxix).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens having a numerical aperture (NA) of 1.0 or greater. Theoretically, it is possible to increase the NA to nearly 1.44. It was initially recognized that the resolution could be degraded and the focus be shifted by a variation of water's refractive index with a temperature change. The problem of refractive index variation could be solved by controlling the water temperature within a tolerance of 1/100° C. while it was recognized that the impact of heat from the resist film upon light exposure drew little concern. There was a likelihood that micro-bubbles in water could be transferred to the pattern. It was found that the risk of bubble generation is obviated by thorough deaeration of water and the risk of bubble generation from the resist film upon light exposure is substantially nil. At the initial phase in 1980's of the immersion lithography, a method of immersing an overall stage in water was proposed. Later proposed was a partial-fill method of using a water feed/drain nozzle for introducing water only between the projection lens and the wafer so as to comply with the operation of a high-speed scanner. In principle, the immersion technique using water enabled lens design to a NA of 1 or greater. In optical systems based on traditional refractive index materials, this leads to giant lenses, which would deform by their own weight. For the design of more compact lenses, a catadioptric system was proposed, accelerating the lens design to a NA of 1.0 or greater. A combination of a lens having NA of 1.2 or greater with strong resolution enhancement technology suggests a way to the 45-nm node (see Proc. SPIE, Vol. 5040, p 724). Efforts have also been made to develop lenses of NA 1.35.
One candidate for the 32-nm node lithography is lithography using extreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUV lithography has many accumulative problems to be overcome, including increased laser output, increased sensitivity, increased resolution and minimized line-edge roughness (LWR) of resist coating, defect-free MoSi laminate mask, reduced aberration of reflection mirror, and the like.
The water immersion lithography using a NA 1.35 lens achieves an ultimate resolution of 40 to 38 nm at the maximum NA, but cannot reach 32 nm. Efforts have been made to develop higher refractive index materials in order to further increase NA. It is the minimum refractive index among projection lens, liquid, and resist film that determines the NA limit of lenses. In the case of water immersion, the refractive index of water is the lowest in comparison with the projection lens (refractive index 1.5 for synthetic quartz) and the resist film (refractive index 1.7 for prior art methacrylate-based film). Thus the NA of projection lens is determined by the refractive index of water. Recent efforts succeeded in developing a highly transparent liquid having a refractive index of 1.65. In this situation, the refractive index of projection lens made of synthetic quartz is the lowest, suggesting a need to develop a projection lens material with a higher refractive index. LuAG (lutetium aluminum garnet Lu3Al5O12) having a refractive index of at least 2 is the most promising material, but has the problems of birefringence and noticeable absorption. Even if a projection lens material with a refractive index of 1.8 or greater is developed, the liquid with a refractive index of 1.65 limits the NA to 1.55 at most, failing in resolution of 32 nm despite successful resolution of 35 nm. For resolution of 32 nm, a liquid with a refractive index of 1.8 or greater and resist and protective films with a refractive index of 1.8 or greater are necessary. Among the materials with a refractive index of 1.8 or greater, the high refractive index liquid seems least available. Such a liquid material has not been discovered because a tradeoff between absorption and refractive index is recognized in the art. In the case of alkane compounds, bridged cyclic compounds are preferred to linear ones in order to increase the refractive index, but the cyclic compounds undesirably have too high a viscosity to follow high-speed scanning on the exposure tool stage. If a liquid with a refractive index of 1.8 is developed, then the component having the lowest refractive index is the resist film, suggesting a need to increase the refractive index of a resist film to 1.8 or higher.
The process that now draws attention under the above-discussed circumstances is a double patterning process involving a first set of exposure and development to form a first pattern and a second set of exposure and development to form a pattern between the first pattern portions. See Proc. SPIE, Vol. 5754, p 1508 (2005). A number of double patterning processes are proposed. One exemplary process involves a first set of exposure and development to form a photoresist pattern having lines and spaces at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying another layer of hard mask thereon, a second set of exposure and development of a photoresist film to form a line pattern in the spaces of the first exposure, and processing the hard mask by dry etching, thereby forming a line-and-space pattern at a half pitch of the first pattern. An alternative process involves a first set of exposure and development to form a photoresist pattern having spaces and lines at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying a photoresist layer thereon, a second set of exposure and development to form a second space pattern on the remaining hard mask portion, and processing the hard mask by dry etching. In either process, the hard mask is processed by two dry etchings.
While the former process requires two applications of hard mask, the latter process uses only one layer of hard mask, but requires to form a trench pattern which is difficult to resolve as compared with the line pattern. The latter process includes the use of a negative resist material in forming the trench pattern. This allows for use of high contrast light as in the formation of lines as a positive pattern. However, since the negative resist material has a lower dissolution contrast than the positive resist material, a comparison of the formation of lines from the positive resist material with the formation of a trench pattern of the same size from the negative resist material reveals that the resolution achieved with the negative resist material is lower. After a wide trench pattern is formed from the positive resist material by the latter process, there may be applied a thermal flow method of heating the substrate for shrinkage of the trench pattern, or a RELACS method of coating a water-soluble film on the trench pattern as developed and heating to induce crosslinking at the resist film surface for achieving shrinkage of the trench pattern. These have the drawbacks that the proximity bias is degraded and the process is further complicated, leading to reduced throughputs.
Both the former and latter processes require two etchings for substrate processing, leaving the issues of a reduced throughput and deformation and misregistration of the pattern by two etchings.
One method that proceeds with a single etching is by using a negative resist material in a first exposure and a positive resist material in a second exposure. Another method is by using a positive resist material in a first exposure and a negative resist material in a higher alcohol of 3 or more carbon atoms, in which the positive resist material is not dissolvable, in a second exposure. However, these methods using negative resist materials with low resolution entail degradation of resolution.
A method which does not involve post-exposure bake (PEB) and development between first and second exposures is the simplest method with high throughputs. This method involves first exposure, replacement by a mask having a shifted pattern drawn, second exposure, PEB, development and dry etching. However, the optical energy of second exposure offsets the optical energy of first exposure so that the contrast becomes zero, leading to a failure of pattern formation. If an acid generator capable of two photon absorption or a contrast enhancement layer (CEL) is used to provide nonlinear acid generation, then the energy offset is relatively reduced even when second exposure is performed at a half-pitch shifted position. Thus a pattern having a half pitch corresponding to the shift can be formed, though at a low contrast. See Jpn. J. Appl. Phy. Vol. 33 (1994) p 6874-6877, Part 1, No. 12B, December 1994. In this regard, if the mask is replaced on every exposure, the throughput is substantially reduced. Then first exposure is performed on a certain number of wafers before second exposure is performed. Due to acid diffusion during the standing time between the first and second exposures, there can occur dimensional variations which are of concern.
The critical issue associated with double patterning is an overlay accuracy between first and second patterns. Since the magnitude of misregistration is reflected by a variation of line size, an attempt to form 32-nm lines at an accuracy of 10%, for example, requires an overlay accuracy within 3.2 nm. Since currently available scanners have an overlay accuracy of the order of 8 nm, a significant improvement in accuracy is necessary.
In addition to the double patterning technique, the technology for forming a fine space pattern or hole pattern includes use of negative resist, thermal flow, and RELACS. The negative resist suffers from the problem that the resist itself has a low resolution. The thermal flow and RELACS methods suffer from a likelihood of variation during dimensional shrinkage by heat.
Referring to FIG. 4, a process for forming a hole pattern using a positive photoresist material. In FIG. 4A, a photoresist material is coated onto a processable substrate 101 on a substrate 100 to form a photoresist film 102. In FIG. 4B, the photoresist film 102 is exposed to light through a photomask having the desired pattern and developed to form a photoresist pattern 102a. In FIG. 4C, the processable substrate 101 is etched while using the photoresist pattern 102a as a mask.
The method of forming a negative pattern by reversal of a positive pattern is well known from the past. For example, JP-A 2-154266 and JP-A 6-27654 disclose naphthoquinone resists capable of pattern reversal. JP-A 64-7525 describes exposure of selected portions of a film to focused ion beam (FIB) for curing and flood exposure whereby the cured portions are left behind. JP-A 1-191423 and JP-A 1-92741 describe exposure of a photosensitive agent of naphthoquinone diazide to form an indene carboxylic acid, heat treatment in the presence of a base into an indene which is alkali insoluble, and flood exposure to effect positive/negative reversal. FIG. 5 illustrates this positive/negative reversal method. In FIG. 5A, a photoresist material is coated onto a processable substrate 101 on a substrate 100 to form a photoresist film 102. In FIG. 5B, the photoresist film 102 is exposed to light through a photomask having the desired pattern and heated. In FIG. 5C, the photoresist film 102 is subjected to flood exposure. FIG. 5D illustrates pattern reversal by development to form a reversed pattern film 103. In FIG. 5E, the processable substrate 101 is etched while using the reversed pattern film 103 as a mask.
As to the positive/negative reversal method including exchange of developers, attempts were made to form negative patterns by development in an organic solvent of hydroxystyrene partially protected with tert-butoxycarbonyl (t-BOC) groups, and by development with super-critical carbon dioxide.
As to the positive/negative reversal method utilizing silicon-containing materials, it is proposed to form a fine hole pattern by covering a space portion of a positive resist pattern with a silicon-containing film, effecting oxygen gas etching for etching away the positive pattern portion, thus achieving positive/negative reversal to leave a silicon-containing film pattern. See JP-A 2001-92154 and JP-A 2005-43420. FIG. 6 illustrates this positive/negative reversal method. In FIG. 6A, a photoresist material is coated onto an undercoat film 104 on a processable substrate 101 on a substrate 100 to form a photoresist film 102. In FIG. 6B, the photoresist film 102 is exposed to light through a photomask having the desired pattern and developed to form a photoresist pattern 102a. In FIG. 6C, the photoresist pattern 102a is crosslinked. In FIG. 6D, a spin-on-glass (SOG) film 105 is formed on the undercoat film 104 and so as to cover the crosslinked photoresist pattern 102a. FIG. 6E illustrates light etching with CMP or CF gas until the crosslinked photoresist pattern 102a is exposed. FIG. 6F illustrates pattern reversal by oxygen or hydrogen gas etching. In FIG. 6G, the processable substrate 101 is etched while using the patterned SOG film 105a as a mask.
As compared with the line pattern, the hole pattern is difficult to reduce the feature size. In order for the prior art method to form fine holes, an attempt is made to form fine holes by under-exposure of a positive resist film combined with a hole pattern mask, resulting in the exposure margin being extremely narrowed. It is then proposed to form holes of greater size, followed by thermal flow or RELACS method to shrink the holes as developed. However, there is a problem that control accuracy becomes lower as the pattern size after development and the size after shrinkage are greater and the quantity of shrinkage is greater. It is then proposed in Proc. SPIE, Vol. 5377, p 255 (2004) that a line pattern in X direction is formed using a positive resist film and dipole illumination, the resist pattern is cured, another resist material is coated thereon again, and a line pattern in Y direction is formed in the other resist using dipole illumination, leaving a grid line pattern, spaces of which provide a hole pattern. Although a hole pattern can be formed at a wide margin by combining λ and Y lines and using dipole illumination featuring a high contrast, it is difficult to etch vertically staged line patterns at a high dimensional accuracy.